I currently fly a T-Rex 450 and want to get a larger heli for Christmas. I'm currently deciding between a 550/600/700.

I haven't done any FPV at all so far, but it is something I'm reading about and will want to try on the heli early next year.

Is there a preferred size for FPV out of a 550 vs 600 vs 700? I've seen people fit all the kit to a 450, but it looks an incredibly tight squeeze and I want something that is easy to work on. How much more space do you get on a 700 to mount FPV equipment, or is the 550 is easily big enough?

I would strongly encourage you NOT to go for anything bigger than a 450 for your first FPV endeavor. Until you have worked out the kinks, a 550 or larger FPV heli is a very dangerous prospect. Not only do you have to get all your gear working correctly and cooperating, but it is very easy to loose perspective of where your heli is and where its going until you get the feel for it. Its not that flying FPV helis is all that hard, its not. But hovering, landing, and keeping the heli from drifting backward or sideways is very challenging in the beginning. It would be very easy to drift backward into yourself or others if you aren't well versed with flying from the FPV perspective. In my opinion newbs flying large FPV helis is extremely dangerous. You can do anything you want with a 450... and the risks are very real if you don't work your way up to flying a large FPV heli.

In my opinion you should not step up beyond a 450 FPV unless you can take off and land your FPV heli from the FPV perspective reliably.

here are my basic rules for FPV helis
1. Always take off and land nose-in so you can see where you are from the camera and avoid hitting yourself.
2. Always fly with a spotter.
3. Learn how to fly without relying on an OSD. they can be helpful, but if you use it as a crutch it can be disorienting and distracting

I would however recommend a stabilization system like CPII when flying FPV for times when the video link goes down. You can switch the auto stab on and it'll save your heli - providing you are flying within LOS and can guide your heli safely to land.

Also, it's good to note that having just a simple FPV system on a 450 is not a tight squeeze at all.

I would start with a 450 with as basic of a setup as possible and move to the 550 when you're comfortable with the 450. You're almost guaranteed to total the thing a few times in the beginning.... better a 450 with cheapo blades than something bigger.

+1 on no bigger than 450 to start off with. I have a 400 I am starting with and then moving it possible to my 500 then 700, I would much rather work out the bugs and range issues on my 250.00 dollar 400 than on my 2K+ 700 bird....